EUV and X-ray optical systems, such as certain types of telescopes, microscopes and lithography systems, generally employ reflective mirrors rather than transmissive lenses due to the short wavelengths involved. The surfaces of such mirrors include a specialized reflective coating that provides a considerable reflectivity coefficient for electromagnetic radiation at the wavelengths of interest.
Two main types of reflective coatings are used: single layer and multiple layers (or “multilayer”). The single-layer coating is typically a thin metal layer (e.g., 100 nm of Au, Ru or Pd film) that has a high reflectivity for low grazing incidence angles α (i.e., where angle α is measured relative to the reflective surface), e.g. >80% for incidence angles a between ˜0° and 18°. Single metal layers are used in so-called grazing incidence mirrors, such as Wolter type mirrors, where the radiation is incident at relatively small grazing incidence angles a, e.g. ˜0° to 20°.
FIG. 1 plots the reflection coefficient R versus grazing incidence angle α for two Ru films. The two Ru films have different thicknesses and were formed using different thin-film deposition methods. The plot shows that the reflectivity R drops considerably for grazing incidence angles a greater than about 20°. Although one of the films displays a consistently higher reflectivity than the other, they both exhibit the characteristic drop off in reflectivity with grazing incidence angle α, which is caused by a fundamental physical effect. Consequently, it is not expected that future advances in the film preparation method will mitigate this effect.
Multilayer coatings typically consist of a periodic or a periodic sequence of thin layers (e.g., Mo and Si layers or films) configured to have a high reflectivity R at some variable angle. Multilayer coatings are used in so-called normal incidence mirrors, where the reflection coefficient R is optimized for light at or near a normal incidence angle φ (i.e., as measured relative to the surface normal). The reflectivity R is tuned by varying the multilayer period. FIG. 2 plots the EUV reflectivity versus normal incidence angle φ for a Mo/Si multilayer coating. A broad peak at about φ=19° is evident.
Common to both types of the aforementioned mirror coatings is the fact that they are configured to be uniform over the spatial extent of the mirror. This restricts the use of the mirror to the limited angular range for which the coating is designed. Consequently, EUV and X-ray optical systems tend to require multiple mirrors, with each mirror coating configured for a different limited angular range. It is desirable to reduce the number of mirrors in such optical systems because each mirror adds to the system complexity and cost, and also decreases the system's overall optical transmission.